A common practice for producing thermoplastics having flame retardant characteristics is to incorporate halogenated compounds. More specifically, aromatic polybrominated compounds have been used as flame retardant additives in such thermoplastic resins. A synergist, which is typically a metal oxide, is applied in combination with the polybrominated compound to achieve better flame retardant characteristics. It is generally accepted that these products inhibit radical gas phase reactions occurring in the flame when ignited. Accordingly, halogenated flame retardants are very commonly used additives for different types of polymeric materials. However, during the last fifteen years or so, halogenated flame-retardants have come under greater scrutiny and some of them have been banned from use because of ecological concerns. At this time, the flame retardant industry is under pressure to change to flame-retardant additives which are more environmentally friendly.
Phosphorus containing products are logical substitutes for such halogenated flame-retardants. In some applications, phosphorus containing additives show at least as much activity as the halogenated ones, although the phosphorus containing additives are less commonly employed. These phosphorus containing additives may provide flame retardant activity through a combination of condensed phase and gas phase reactions (E. D. Weil, W. Zhu, N. Patel and S. M. Mukhopadhyay, Polymer Degradation and Stability, 1996, vol. 54, pp. 125-136). The phosphorus containing products are able to react with the polymer during combustion and increase its carbonization while reducing the production of volatile combustible gases. These processes obviously depend on the polymer in which such additive(s) are used. Therefore, specific phosphorus containing structures must be tailored for each type of polymer. The gas phase action of the phosphorus containing additives is more generic; since it does not depend on the polymer type.
The description of the use of a combination of two or more phosphorus containing flame retardant additives in thermoplastic resins is relatively extensive in the prior art. For example, U.S. Pat. Nos. 4,257,931 and 5,814,690, (the contents of each are herein incorporated by reference) Great Britain Patent Publication No. 2,330,583, German Patent Publication No, 44,26,010, French Patent Publication No. 2,781,807, Japanese Patents Nos. 10,287,761, 10,316,813, 11,116,817, 11,181,199 and 11,302,656, European Patent Publications Nos. 897,413, 896,023, 899,296 and 947,560, and PCT International Patent Publications Nos. WO 98/39 381, WO 98/53 002, WO 99/27 016 and WO 99/28 382 all describe combinations of two or more phosphorus containing flame retardants.
However, the combinations disclosed in the prior art use relatively large amounts of such phosphorus containing additives. Some of these additives have limited solubility in polymers, especially in semi-crystalline polymers and therefore tend to migrate to the polymer surface. This migration phenomenon adversely affects the surface appearance and the combustion performance of the polymer product. If the phosphorus-containing additive or combinations of additives are soluble in the polymer, they tend to plasticize the polymer and reduce the thermal properties, such as, heat distortion temperature of the polymer. Numerous attempts to rectify these problems by copolymerization of phosphorus containing units into the polymer chains have been attempted but have only had low to moderate commercial success (see, for example, E. D. Weil, Phosphorus-Containing Polymers, in the Kirk-Othmer Encyclopedia of Polymer Science and Engineering”, Vol. 11, John Wiley, New York, 1990, pp. 96-126). The reason these compounds were not commercially successful was because of their high cost and a decrease in the physical properties of the resulting polymer composition.
Another approach described in the prior art relates to the combination of a high charring polymer and a phosphorus-containing additive, which is normally compatible with the high charring polymer. For example, U.S. Pat. No. 6,569,928, the contents of which are herein incorporated by reference, describes the use of a high charring polymer that contains benzene rings in the main polymer backbone in combination with phosphorous containing additives. This combination of additives is applied to a semi-crystalline polyester, which prevents migration of the phosphorus-containing additive to the surface as compared to when the phosphorous containing additive is used without combining it with a high charring polymer. However, the flame retardant compositions described in this patent are limited to polyesters and the organic additive must have benzene rings incorporated directly into the backbone of the organic phosphate. Relatively high concentrations of the flame retardant is also required and therefore do not make the composition a commercial alternative to other existing products on the market.
Still another approach described in the prior art relates to the combination of a high charring polymer with metal alkyl-alkyl phosphonate as a flame retardant. For example, European Patent Publication No. 0,356,634 describes the use of high-impact polyphenylene ether comprising aluminum-methyl-methyl phosphonate as the flame retardant and a fatty acid having 12-20 carbon atoms or a salt thereof. The fatty acid is used to increase the impact strength of articles manufactured from the resulting composition without significantly influencing the flame retardant properties of the manufactured article. This European patent further discusses optionally intensifying the flame retarding activity of the fatty acid containing composition by adding a tetra-arylphenylene diphosphate.
U.S. Pat. Nos. 6,258,879 and 6,486,244, the contents of each are herein incorporated by reference, disclose a process for manufacturing concentrates of polyphenylene ether resin and an organic phosphate. This concentrate is eventually ground to a powder containing less than 5% by weight of particles less than about 75 micrometers in size. This grinding process helps to decrease the amount of energy required for compounding and avoids the explosion risk associated with the handling of potentially airborne powder. These compositions also often require a high phosphate content to exhibit acceptable flame retardant characteristics.
In view of the foregoing, what is needed is a flame retardant composition that can be blended with a thermoplastic resin to produce a flame retarded thermoplastic composition that is more environmentally acceptable, has high flame retardant characteristics, does not have components that migrate to the surface of the polymer requires less loading of the phosphate material so as to make it more economically attractive than other thermoplastic compositions available on the market today. The halogen-free or substantially halogen-free flame retardant composition and the resulting flame retarded thermoplastic composition of the present invention possesses these characteristics as well as others and therefore overcome the problems associated with the flame retardant compositions described in the prior art.